1. Field of the Invention
The present invention generally relates to brushless, sensorless, DC motor controllers. Specifically, the present invention relates to a completely digital logic implementation of a brushless, sensorless DC motor controller that uses back electromotive force values from the DC motor to selectively switch power to windings in the DC motor stator.
2. Background of the Invention
In conventional brushless DC motors, a displacing force (rotational or linear) is generated in the movable rotor portion of the motor by inducing a current in the stator windings. This stator winding current creates a stator magnetic field that causes the armature field in the motor rotor to attempt to align with the induced stator field. If a constant DC voltage is applied to a single coil or winding in a two-pole stator, the rotor will rotate and come to rest with the rotor poles aligned with the induced stator poles. To maintain rotor displacement, the current in the stator winding must be reversed to switch the polarity of the stator poles. If timed correctly, the momentum of the rotor and the switched magnetic field will act to keep the rotor turning.
Maximum torque is developed when the rotor field is normal to (90° ahead of) the stator field. If the polarity of a two-pole stator field is switched as the rotor armature field approaches alignment with the stator field, the rotor and stator fields can be as much as or over 180° apart. Thus, a simple DC motor comprising two stator poles will exhibit large torque variations as the rotor moves. Improvements to this basic, two-pole brushless DC motor model have been implemented by incorporating more stator coils, windings, and poles and a corresponding increase in rotor poles. Three-phase brushless motors are commonly used for this type of application because the separate phases of a three-phase power supply can be coupled to multiple stator windings to induce multiple stator poles. An increase in the number of stator poles in a DC motor offers the benefit of producing a more efficient motor. However, the additional poles also increase motor complexity because stator winding current must be switched on and off to keep the stator poles ahead of the rotor poles as the rotor moves with respect to the stator. This switching is often described as commutation, which refers to the synchronous regulation of stator and rotor magnetic fluxes.
At least two predominant approaches have been used to successfully implement this type of switching. Older, more conventional DC motors used a brush-commutator system to continuously switch drive current as the rotor moved. In these motors, motor voltage was applied via physical contact between the brushes and winding contacts on the rotor armature. These winding contacts are typically separated by gaps where the brush contacts do not contact any windings. Thus, as the rotor moves, the brush contacts lose contact with a first winding before making contact with a second winding. With this functionality, motive power can be applied to the armature windings in the correct sequence to maintain rotor motion. While effective, these brushes were subject to mechanical wear over time. Furthermore, this configuration added to the amount of hardware and wiring that was needed to successfully drive the motor.
As an alternative approach, brushless DC motors were subsequently developed to eliminate brush contacts. In these latter designs, the automatic switching effect provided by the brushes in older conventional motors had to be accounted for. To accomplish this, brushless DC motor controllers were developed to control the timing and switching of power delivery to the motor. In addition, the motor configuration is changed to move the switched windings from the rotor to the stator. Further, the permanent magnets are transferred to the rotor. In practice, the motor controllers incorporate feedback signals that provide the precise location of the rotor with respect to the stator at any given time. In conventional systems, this position information is provided using hall effect sensors, optical sensors, or synchro resolvers. Signals from these external sensors are fed back to the motor controller, which switches stator windings on and off (thereby moving the induced magnet field) based on the position of the rotor. These brushless DC motor solutions significantly reduced, if not eliminated, the conventional mechanical wear problem, but they still required extra hardware and wiring to provide the necessary position feedback.
Accordingly, another family of brushless DC motor controllers was developed that use back EMF signals generated by the motor windings to indicate rotor position. The motor configuration for these brushless, sensorless DC motors remains the same as the sensored, brushless DC motors. That is, the drive windings are located in the stator and the permanent magnets are disposed about the rotor.
It is well known to those skilled in the art that Faraday's Law necessitates that a moving magnetic flux in the presence of a conductor will induce a voltage in that conductor. This voltage is commonly referred to as electromotive force (EMF). The EMF phenomenon permits a permanent magnet DC motor to work as a generator. Thus, in a brushless, sensorless DC motor with multiple stator windings, a voltage naturally appears across the stator windings as the rotor moves. In the motor context (as distinguished from a generator), the EMF induced in the stator windings is a byproduct of the drive power supplied by the motor controller and power supply. As such, this EMF may be classified as Back EMF. The magnitude of this Back EMF changes as the permanent magnets move towards and away from the stator windings. As such, the magnitude of the Back EMF signal can be used to indicate position of the rotor. Various methods are used in the art to translate the Back EMF signal into a position indication, including comparators, accumulators, and perhaps even simple switches.
In practice, a brushless, sensorless DC motor will incorporate a plurality of stator windings and use a motor controller to synchronously switch power to the stator windings to induce motion in the rotor. A common configuration includes a stator with three windings and multiple rotor magnet poles. To induce motion in the rotor, two of the stator windings are driven, while the third winding is undriven, or essentially off. The Back EMF levels in the third winding can then be measured and used to identify the position of the rotor. As the motor controller switches power to the DC motor, the third winding will be turned on (driven) and one of the previously driven windings will be turned off. Naturally, the process repeats during normal motor operation.
The advent of brushless, sensorless DC motor controllers that use Back EMF signals to indicate rotor position has eliminated the need for commutator brushes, revolvers, and hall effect sensors to indicate the position of a rotor as it moves with respect to the stator. The Back EMF signals are incorporated into a feedback loop between the DC motor and the DC motor controller to correctly time the switching of power to the motor windings. Thus, the heart of the operation lies with the motor controller and any methods used by the controller to interpret the Back EMF signals to correctly time the switching events.
One problem with using Back EMF signals to indicate rotor position is that the rotor must be moving to generate any Back EMF signal at all. Thus, most conventional brushless, sensorless DC motor controllers incorporate some functionality to initiate rotor movement and thereby generate the required Back EMF signals.
Prior art brushless, sensorless DC motor controller devices exist in a variety of configurations. As brushless, sensorless DC motors become more prevalent, off-the-shelf motor controllers have become more common. As an example, Philips Semiconductors offers dedicated bipolar integrated circuits (IC) to drive three phase brushless DC motors. Similarly, Texas Instruments has proposed the use of a digital signal processor (DSP) to implement a software motor controller. These prior art solutions have been proven to work effectively in controlling DC motor commutation. However, there are a number of drawbacks and limitations to these prior art motor controllers.
First, any controllers that use bipolar technologies are limited to use in reasonable operating temperatures. Conventional bipolar devices can operate effectively up to about 70° C. In addition, some ICs require external energy storage devices such as capacitors to control critical timing. These additional devices also lack long-term reliability at elevated operating temperatures. Operating temperature range can also be prohibitive in processor designs, with 70° C. also being a typical maximum operating temperature for processors and DSPs. Military grade DSPs offer a maximum operating temperature in the 120°-130° C. range, but this may still limit applicability to high temperature (in excess of 150° C.) environments, such as those found in proximity to combustion engines or downhole drilling applications.
A second problem with prior art brushless, sensorless DC motor controllers is complexity and reliability. As mentioned above, ICs often require analog components to control commutational timing. These analog components tend to inhibit the long-term reliability of these designs. Other designs employing processors are even more complex and expensive. A typical DSP may include multiple processors, memory devices, and memory controllers and require software code to be written, debugged, and integrated before deployment. This complexity often hampers time to market. Further, it is generally accepted that designs that are more complex exhibit lower long-term reliability.
Therefore, a primary consideration in improving reliability is reducing complexity of the motor controller. Further, it is also desirable to create a fully digital embodiment of a brushless, sensorless DC motor controller such that the controller may be implemented on a single digital logic device. A fully digital embodiment may also permit incorporation onto a high temperature digital logic device, such as a semiconductor on insulator (SOI) device. SOI devices can incorporate digital logic components mounted onto a thin, single-crystal semiconductor film growing atop a synthetic insulator substrate. The insulator may be silicon oxide, glass, or sapphire. SOI devices were originally developed for use in military and aviation applications requiring high temperature and “radiation hardened” chips.
Thus, a simple, fully digital implementation of a brushless, sensorless DC motor controller offers distinct advantages over prior art designs. The improved digital design may be implemented on an off-the-shelf programmable logic device costing only a few dollars. Such a design may advantageously provide a low-cost, reliable alternative to existing controller designs for use at reasonable operating temperatures. In addition, the improved digital design may be implemented on SOI logic devices to provide a reliable, high-temperature motor controller solution that is not available with conventional controllers. Further, the new controller requires a single clock, the frequency of which may be adjusted to accommodate a range of BLDC motors. Existing controllers are often difficult to tune and may require the changing of analog components or program code.
It is therefore desirable to develop a fully digital embodiment of a brushless, sensorless DC motor controller that uses Back EMF feedback information from the DC motor to accurately commutate the DC motor. The motor controller preferably provides a simple, reliable alternative to conventional motor controllers. In addition to proper commutation, the improved motor controller should effectively account for startup conditions where Back EMF signals are not present. Further, the improved motor controller should advantageously adapt to varying input voltages and rotor velocities.